Plating system



United States Patent 3,408,211 PLATING SYSTEM Harold Jefferson Coates, Henrico County, Va., assignor to Reynolds Metals Company, Richmond, Va., a corporation of Delaware No Drawing. Filed Feb. 9, 1966, Ser. No. 526,025

9 Claims. (Cl. 106-1) ABSTRACT OF THE DISCLOSURE The immersion plating of lead or antimony on aluminum and aluminum base alloys is carried out in a bath comprising an aqueous solution of an alkali soluble salt of lead or antimony, an alkali metal tartrate, an alkali soluble zinc compound, a ferric salt, and sufficient alkali metal hydroxide to render the bath alkaline. This bath enables removal of the oxide layer present on the aluminum surface by alkali, replacement by a temporary layer of zinc, followed by replacement of the zinc layer by the lead or antimony coating.

This invention relates to a novel method and composition for the immersion plating of lead or antimony on aluminum and aluminum base alloys. More particularly,

'the invention concerns an immersion plating system wherein the aluminum oxide layer present is removed with an alkali and replaced by a temporary layer of zinc, followed by replacement of the zinc layer by the lead or antimony coating.

The favorable conductivity and strength of aluminum and aluminum base alloys have led to increased applications in the automotive field, particularly for radiator core and fin stock. Owing to corrosive conditions it is desirable to apply to the aluminum metal a protective metal coating. Lead or antimony is commonly employed for this purpose, but it has been difiicult to obtain dependably adherent coatings of these metals on the aluminum. Chemical plating or immersion plating has been used, and it is a prerequisite that the surface of the aluminum must be free from impurities such as aluminum oxide film which form by air oxidation or in the plating bath. Aluminum is prone to rapid oxidation which forms a hard film that must be removed before plating. If this film is not removed, the plating bridges over the oxide portion or over other impurities on the aluminum surface so that the plating only mechanically adheres to that portion of the surface of the aluminum containing the oxide film. Such bridging results in a plated surface that peels, blisters or cracks, particularly when the surface is subjected to strains such as are produced by temperature elongation. The removal of the aluminum oxide film is usually achieved by an etching treatment in a strong alkali solution, such as sodium hydroxide, followed by washing, and immersion of the metal in the plating bath.

It is known to immersion plate lead and lead-antimony alloys on aluminum from an aqueous bath containing an alkaline tartrate complex and employing a lead compound which is soluble in the alkaline bath, such as lead monoxide or lead acetate. It is also known to electroplate zinc on aluminum from an alkali metal zincate bath in presence of a soluble ferric salt, such as ferric chloride.

In accordance with the present invention, the chemical plating of lead or antimony on aluminum and base alloys is accomplished by employing as a plating bath an aqueous mixture of a lead salt or antimony salt, as the case may be, an alkali metal tartrate, an alkali-soluble zinc compound, a ferric salt, and a sufficient amount of an alkali metal hydroxide to render the mixture alkaline. Under these conditions, the immersion plating bath of the invention acts to dissolve any aluminum oxide layer which may be present on the surface of the metal to be plated and 'ice to coat the aluminum with a thin layer of zinc. Although both lead and antimony are lower in the electromotive series than zinc, it has been found, surprisingly and unexpectedly, that under these conditions, the zinc layer is in turn displaced by the lead or the antimony, to form a hard coat of either of these metals.

The lead compound employed in accordance with the invention is one which is alkali soluble, such as, for example, lead monoxide, basic lead carbonate Pb(OH) (PbCO or acetate.

The antimony compound is also one which is alkali soluble, such as, for example, potassium antimony tartrate.

The lead and antimony compounds are employed in a concentration ranging from about 0.5% to about 2.0% by weight of the bath, preferably about 1.0% by weight.

The ferric salt employed is preferably ferric chloride, in a concentration ranging from about 0.05% to about 0.1% by weight of the bath.

The alkali hydroxide employed may be, for example sodium or potassium hydroxide, sodium hydroxide being preferred. The concentration ranges from about 20% to about 60% by weight of the bath.

The ferric salt employed is preferably ferric chloride, in the bath in an amount from about 2.5% to about 12.5% by weight, preferably between about 5% and 10%.

The tartrate complex salt may be, for example, sodium potassium tartrate, present in an amount from about 0.5% to about 1.0% by weight of the bath.

The plating is advantageously carried out at slightly elevated temperature, for example between about and F., but temperature is not critical.

There is also included in the plating baths of the invention an amount of chelating agent suflicient to control the action of the ferric salt by forming a complex with the iron to solubilize it and to prevent interference with the zinc, or lead or antimony deposition, suitable chelating agents include ethylene diamine tetra acetic acid (EDTA) and its salts, e.g., the mono-, di-, trior tetrasodium salt.

Prior to plating, the aluminum or aluminum base alloy article is conditioned by degreasing, for example in a dilute 5% sodium hydroxide solution, rinsed with water, and then subjected to surface activation by means of an acid dip, such as a 10 second dip in a mixture of 3% hydrofluoric acid and 10% nitric acid, followed by a final rinse.

The following examples illustrate the practice of the invention, but are not to be regarded as limiting:

EXAMPLE 1 Lead carbonate g./l 1O Versene Na-EDTA (triacid) ml./l.. 250 Zinc oxide g./l 50 Sodium hydroxide g./l 250 Sodium potassium tantrate ..g./l 5 Ferric chloride' g./l 0.5

The sample was then rinsed with water and dried in air.

EXAMPLE 2 As in Example 1, a section of radiator core stock was immersed for one minute in an antimony plating bath having the following composition:

G./l. Zinc oxide Sodium hydroxide 525 Sodium potassium tartrate 10 The sample was then rinsed with water and dried in air.

What is claimed is:

1. A bath for the immersion plating of lead or antimony on aluminum and aluminum base alloys, comprising an aqueous solution of from about 0.5% to about 2.0% of an alkali soluble salt of a metal selected from the group consisting of lead and antimony, from about 0.5% to about 1.0% of an alkali metal tartrate, from about 2.5% to about 12.5 of an alkali soluble zinc compound, from about 0.05% to about 0.1% of ferric chloride, all percentages being by weight, and a suflicient amount of an alkali metal hydroxide to render the bath alkaline.

2. The bath of claim 1 in which the lead salt is lead carbonate.

3. The bath of claim 1 in which the antimony salt is potassium antimonyl tartrate.

4. The bath of claim 1 in which the zinc compound is zinc oxide.

5. The bath of claim 1 in which the ferric salt is ferric chloride.

6. The bath of claim 1 in which the alkali metal hydroxide is sodium hydroxide.

7. Method for chemically plating on aluminum and aluminum base alloys a metal selected from the group consisting of lead and antimony, comprising immersing the aluminum in an aqueous solution of from about 0.5 to about 2.0% of an alkali soluble salt of a metal selected from the group consisting of lead and antimony, from about 0.5% to about 1.0% of an alkali metal tartrate, from about 2.5% to about 12.5% of an alkali soluble zinc compound, from about 0.05% to about 0.1% of ferric chloride, all percentages being by weight, and a suflicient amount of an alkali metal hydroxide .to render the bath alkaline, at a temperature between about 75 and 85 F., whereby there is first deposited on the aluminum a thin layer of zinc, which is in turn displaced by the lead or antimony, forming a hard coat of said lead or antimony.

8. The bath of claim 1 which further includes a chelating agent.

9. The bath of claim 8 in which the chelatin agent is a sodium salt of ethylene diamine tetra-acetic acid.

References Cited UNITED STATES PATENTS 2,813,805 ll/1957 Kendall et al 106-1 XR 3,050,410 8/1962 Greene 1061 XR 3,216,835 11/1965 Saubestre 1061 JULIUS FRCME, Primary Examiner.

ALEXANDER H. BRODMERKEL, Examiner.

L. B. HAYES, Assistant Examiner. 

